The deposition of gold on selected surfaces of electronic circuitry by chemical or electroless processes has been actively investigated for several decades. There are many reviews that thoroughly describe these efforts. (Electroless Plating, Y. Okinaka, p. 401-419 (1990); H. O. Ali and I. R. A. Christie, Gold Bull., 17 (4), 118 (1984)). There are three mechanisms by which a water-soluble gold salt can be deposited on a metallic substrate. They are:
(1) A displacement process where aurous (or auric) gold is reduced to metallic gold and the substrate metal, usually nickel, is oxidized to the plus two valence state as a water soluble complex. PA1 (2) A catalytic displacement process where a metal, such as palladium, is deposited on the metal substrate and the palladium promotes the reduction of aurous gold to metallic gold and the oxidation of the substrate metal to a metal ion., PA1 (3) A catalytic chemical process whereby a water-soluble gold complex is reduced by a reducing agent present in the composition and metallic gold is deposited on a gold surface which acts as a catalyst for the chemical deposition. PA1 1. 3-mercapto-1-propanesulfonate, sodium salt (R=--CH.sub.2 CH.sub.2 CH.sub.2 SO.sub.3 Na) PA1 2. 2-mercaptoethane sulfonate, sodium salt (R=--CH.sub.2 CH.sub.2 SO.sub.3 Na) PA1 3. 3-mercapto 2-hydroxy-1-propanesulfonate, sodium salt (R=--CH.sub.2 CH(OH)CH.sub.2 SO.sub.3 Na) PA1 4. mercaptosuccinic acid (or thiomalic acid, R=--CH(COOH)CH.sub.2 COOH) PA1 5. mercaptoacetic acid (R=--CH.sub.2 COOH) PA1 6. 2-mercaptopropionic acid (R=H.sub.3 C--(CH)--CH.sub.2 COOH) PA1 7. thiosalicylic acid (R=--CH.sub.6 H.sub.4 COOH) PA1 8. L-cysteine (R=--CH(NH.sub.2)COOH)
The disclosures of the invention are concerned specifically with a catalytic electroless gold deposition process as defined in (3) above. Electroless gold baths of this type were first patented by Okinaka (U.S. Pat. No. 3,700,469 (1972)). These baths contain potassium aurous cyanide as the source of gold, potassium or sodium cyanide as a stabilizer, and an alkali metal borohydride or dimethylamine borane as the reducing agent. Alkali metal hydroxides such as potassium hydroxide provide the alkalinity for the borohydrides to function effectively as reducing agents. The borohydride baths, when handled carefully, gives useful results. However, they have several serious shortcomings. Traces of nickel ions and or organic impurities result in bath decomposition. The initial plating rate in practice, is slow, about 1 micron/hour. As the bath continues to be used, the cyanide ion concentration increases slowing down the deposition rate. Another shortcoming of the Okinaka bath is the formation of small gold granules in the plating solution. Extended use of the bath by replenishment decreases its stability and slows down the deposition rate. For practical applications, a gold deposition rate of two to five microns per hour is required. Trace amounts of lead ions or thallous ions will enhance gold plating rates. (M. Matsuoka, S. Imanishi, M. Sahara, and T. Hayashi, Plating and Surface Finishing, p. 102 (1988)). Although 1-2 mg/liter of thallous ion significantly increases the gold deposition rate, it has a negative effect on the overall stability of the plating bath. The use of trace amounts of lead in this bath requires careful monitoring because of the incorporation of this ion in the deposit and the adverse affect it might have on its functional properties.
Another process is described by F. Simon, Gold Bull., 26(1) pp. 14-23 (1993), wherein an electroless bath utilizes potassium aurous cyanide. At pages 21-22 of this reference, the author describes the inhibitory effect of accumulating cyanide ion on the rate of gold deposition and a vague reference to the addition of an accelerator to overcome this slowdown.
The stabilization of electroless gold baths based on borohydride/amineborane reducing agents, potassium aurous cyanide, alkali metal cyanides, and alkali metal hydroxides has been a major problem in the use of this chemistry. Trace amounts of transition metal ions, organic impurities, and localized over heating are among the contributors leading to the spontaneous decomposition of the bath.
The catalytic electroless gold bath of this invention is a quasi-stable solution which maintains its integrity as long as certain electrochemical and chemical conditions are met. The gold source utilized in the bath of the present invention does not contain cyanide and therefore gold deposition and replenishment cannot increase the cyanide concentration above the amount needed to stabilize the bath. The catalytic nature of the gold substrate permits the adsorption of preferred intermediates formed by the interactions of the chemical agents that make up the solution which in turn lead to the deposition of metallic gold on the substrate. If this sensitive process is interfered with by impurities, unfavorable electrochemical conditions, or excessive temperatures, bath decomposition will result, frequently instantaneously.
To protect these baths against trace metals, the use of chelating agents have been proposed. Where the decomposition is due to nickel ions, these chemicals at best slow down the decomposition but do not prevent it. European Patent Application No. 0 343 816 discloses a potassium borohydride/potassium aurous cyanide bath containing potassium ferrocyanide and potassium ferricyanide which can plate gold directly on nickel without decomposition. Monoethyl ethers of ethyleneglycol and diethylene glycol have been disclosed as stabilizers, (DE Patent No. 3707817 AI)as well as ethyleneglycol, (U.S. Pat. No. 4,919,720).
The accumulation of cyanide ions in the bath due to the plating process slows down the rate of gold deposition. (Y. Okinaka and C. Wolowodiuk, Plating, 58, 1080 (1971)). These investigators disclosed the use of aurous cyanide as a gold replenishment to partially offset the inhibitory effect of increasing cyanide concentration on the deposition rate. An alternative method to avoid the accumulation of ionic cyanide is given in U.S. Pat. No. 3,917,885 where an aqueous solution of an alkali metal gold succinimide complex is used instead of potassium aurous cyanide. Potassium cyanide is included as a stabilizer. No details are given for the preparation of the gold succinimide complex but it is probable that the gold is in a trivalent state because compounds of this type are prepared from potassium tetrachloroaurate. A related approach is disclosed in U.S. Pat. No. 4,337,091 where potassium aurate (or auric hydroxide)is used as the gold source.
One of the products resulting from the alkali metal borohydrides and dimethylamine borane is potassium metaborate. The accumulation of this chemical in the bath causes the precipitation of gold particles and is responsible for a loss of the gold plating component. It is a common practice in gold electroplating to dissolve gold deposits by the use of a water soluble aromatic nitro compound and an alkali metal cyanide. This chemistry is used in DE Patent No. 3938653 AI to prevent the formation of gold particles in their electroless plating bath.